Operator overloading << error: must take exactly one argument

[nomiz]

Hi all!

I can't get my head around the compile error I get when overloading the << operator:
error: ‘QDataStream& Coordinate:perator<<(QDataStream&, const Coordinate&)’ must take exactly one argument

Here is the code:

coordinate.h:

class Coordinate
{
public:
    Coordinate();
 
    double x, y, z;
 
    // output
    QDataStream &operator <<(QDataStream& out, const Coordinate& c);
};

class Coordinate
{
public:
    Coordinate();

    double x, y, z;

    // output
    QDataStream &operator <<(QDataStream& out, const Coordinate& c);
};

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coordinate.cpp:

#include "coordinate.h"
 
Coordinate::Coordinate()
{
    x = y = z = 0.;
}
 
QDataStream& operator <<(QDataStream& out, const Coordinate& c)
{
    out <<  "(" <<  c.x << ", " << c.y << ", " << c.z << ")";
    return out;
}

#include "coordinate.h"

Coordinate::Coordinate()
{
    x = y = z = 0.;
}

QDataStream& operator <<(QDataStream& out, const Coordinate& c)
{
    out <<  "(" <<  c.x << ", " << c.y << ", " << c.z << ")";
    return out;
}

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What is wrong / am I not getting?

Thanks!

[wysota]

The operator should be a standalone function and not a method of your class.

[Zlatomir]

If you want to make the data members private you can add friend operator inside your class:

class Coordinate
{
public:
    Coordinate();
    // output
    friend QDataStream &operator <<(QDataStream& out, const Coordinate& c);
private:
        double x, y, z;
};

class Coordinate
{
public:
    Coordinate();
    // output
    friend QDataStream &operator <<(QDataStream& out, const Coordinate& c);
private:
        double x, y, z;
};

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[nomiz]

The operator should be a standalone function and not a method of your class.

Worked! Thank you!

Why does operator== not give any troubles?

If you want to make the data members private you can add friend operator inside your class:

Ok, thanks for your info!

[wysota]

Worked! Thank you!

Why does operator== not give any troubles?

If you put the operator inside the class definition, then the operator takes the current object ("this") implicitly as its first parameter.

So operator== comparing A against B can be placed in class A but not in B (or it can be a standalone function taking two parameters -- A and B). If you have a standalone operator << that takes QDataStream and MyStruct as its parameters, you could put it in QDataStream class but not in MyStruct class. Since in C++ you can't modify a class that has already been defined, you can't put it in QDataStream class and you're left with the standalone option.

[nomiz]

Aha! Thanks alot. I'll buy and read a proper C++ book

[wysota]

That's usually a good idea